1. Technical Field
The present invention relates to image forming apparatuses, such as electrophotographic copy machines, printers, and facsimile machines.
2. Description of the Related Art
In a conventional image forming apparatus, an image transferred from an image carrier onto a recording medium is fused onto the recording medium using a combination of heat and pressure, thus obtaining a copied or recorded material. During the fusing process, the recording medium is typically pressed and held between a fusing roller and a pressure roller while the un-fused image on the recording medium is heated. In this way, a developing material in the un-fused image, such as toner, is caused to melt and penetrate the recording medium, thus fixing the toner onto the recording medium.
In such an image forming apparatus, the thickness of the core of the fusing roller may be reduced to reduce the heat-storing effect of the roller in order to reduce power consumption during a warm-up period. Power supplied to a heat source may also be reduced. However, in an image forming apparatus that requires a high processing speed, such as copy machines, the amount of heat transferred to the recording medium may be greater than the amount of heat supplied to the fusing roller from a heat source (such as a halogen heater). In this case, the temperature of the fusing roller decreases during a continuous copy operation.
When the temperature of the fusing roller or the pressure roller is lower than a certain minimum temperature, a sufficient amount of heat cannot be supplied to the recording medium. As a result, the recording medium is passed via a fusing nip formed between the fusing roller and a fusing belt, for example, before the toner is sufficiently melted, thus failing to fuse the toner onto the recording medium properly.
In order to obtain required fusing properties, a “CPM (copy per minute) down” control may be performed. In this operation, when the temperature of the fusing roller or the pressure roller drops below a certain value, the intervals of sheets of the recording medium as they are fed to the fusing nip are increased so that a sufficient time can be ensured for the fusing roller or the pressure roller to be sufficiently heated for the fusing operation.
Japanese Laid-Open Patent Publication No. 2007-226028 (“Patent Document 1”) discloses a technology for eliminating control error immediately after reload and for improving fusing quality. In this technology, a sheet size and a paper type are detected upon entry into a copy operation from a standby status, and then a number of sheets for starting CPM down control (which may be referred to as a “CPM down control starting number”) is determined. The CPM down control starting number is then corrected by detecting the temperature of the fusing roller at the start of feeding a sheet. The number of sheets that have been continuously fed is counted, and the CPM down control starting number is modified depending on the number of continuously fed sheets.
Japanese Laid-Open Patent Publication No. 57-67970 (“Patent Document 2”) discloses that power supply variations are checked and a decrease in fusing temperature and its recovery time are predicted based on the power supply variations and a set number of copied sheets. The copy rate per unit time may be decreased in advance in accordance with the prediction.
However, the conventional fusing error preventing measures have not been optimized for the heating power of the particular fusing apparatus or the number of sheets fed. As a result, the fusing apparatus may be heated longer than necessary, thereby extending the wait time for the user or the time required for printing a number of sheets, resulting in a decrease in productivity.